Solution : https://service.sap.com/sap/support/notes/207835 (SAP Service marketplace login required)
Key words : 
additional key words saplfaci, final billing document, vendor net procedure, cleared gross, payment clearing, implemented notes 181183, program correction, advance correction, prerequisites
Related Notes : 
       
| 645352 | AC interface: BSEG-XANET indicator set incorrectly | 
| 211001 | Invoices:Balance when posting down payment clearing (net) | 
| 206622 | Error when clearing down payment request in net procedure | 
| 181183 | Down payment account tax category is ignored | 
| 113347 | Down payment process. as of Release 4.0x (standard) |